Recently, general interest in environmental issues is on the increase, and, regarding, for example, automobiles, there is a strong demand for an enhancement in emission control, an improvement in fuel efficiency, etc. In this context, there is a strong demand for a further reduction in the weight and an enhancement in the strength of power transmission shafts to be used as drive shafts, propeller shafts, or the like. Many of those power transmission shafts have a spline section in the outer peripheral surface thereof. Through fit-engagement of the spline section (male spline section) of a power transmission shaft and the spline section (female spline section) of a female member to be fitted onto the power transmission shaft, the power transmission shaft and the female member are connected together, and rotational power is transmitted.
There is required high strength of a power transmission shaft having a male spline section, and hence, usually, steel is used as the material. After forming the male spline section by rolling, press working, or the like, at least the male spline section is hardened for use through quenching. The female spline section is often formed through broaching or the like. As the method of hardening after forming, induction hardening through quenching is often adopted. In some cases, immersion quenching, carburizing and quenching, or the like is adopted.
FIG. 8 is a plan view of a so-called round up type male spline section in which the end portion opposite to the shaft end (left-hand side in the drawing) of a valley section 100 is connected to the outer peripheral surface (smooth portion) 101 of a male member via a diameter-expanded section 102 whose outer diameter is gradually increased. Fatigue fracture of a spline section of this type usually occurs near the connection between the valley section 100 and the diameter-expanded section 102 or in the diameter-expanded section 102. In this case, there are two modes in which a crack is generated: one due to tensile stress concentrated on portion A, and one due to shear stress concentrated on portion B. In the case of steel, when the Vickers hardness is less than 700, shear stress is predominant. When the Vickers hardness is 700 or more and in the case of pulsating torsional fatigue, tensile stress is predominant.
Up to now, several methods have been proposed as means for achieving an improvement in terms of the fatigue strength of a spline section. For example, Patent Document 1 discloses a technology for mitigating stress concentration by blunting the boundary between a diameter-expanded section and a tooth surface. Patent Document 2 discloses a strength enhancing technology according to which, instead of arranging one diameter-expanded section as usual, two or more diameter-expanded sections are arranged side by side.    Patent Document 1: JP 2005-147367 A    Patent Document 2: JP Hei 11-514079 A